This invention relates to a reflective mask blank for exposure, a reflective mask for exposure, and a multilayer reflective film-coated substrate for use in the semiconductor production or the like and to a method of producing a semiconductor device.
In the semiconductor industry, use has conventionally been made of the photolithography method using visible light or ultraviolet light as a transfer technique of a fine pattern required for forming a fine-pattern integrated circuit on a Si substrate or the like. However, wavelength shortening in the conventional light exposure has been approaching the exposure limit while the miniaturization of semiconductor devices has been accelerated. The pattern resolution limit is said to be ⅓ of an exposure wavelength in the case of the light exposure and, for example, it is about 60 nm using an ArF excimer laser (wavelength 193 nm). In recent liquid immersion exposure, an improvement in resolution has been achieved and it is expected that about 45 nm can be resolved, but a further increase in light-exposure resolution is difficult. In view of this, as an exposure technique with a higher resolution than 45 nm, the EUV lithography (hereinafter referred to as “EUVL”) being an exposure technique using EUV light with a wavelength shorter than that of the ArF excimer laser is expected to be promising. Herein, the EUV (Extreme Ultra Violet) light represents light in a wavelength band of the soft X-ray region or the vacuum ultraviolet region and, specifically, light having a wavelength of about 0.2 to 100 nm.
The image forming principle of EUVL is the same as that of the photolithography. However, since the absorption of any substance for EUV light is large and the refractive index is close to 1, use cannot be made of a refractive optical system used in the light exposure, but is made of a reflective optical system in any case. As a mask for use in that event, there has conventionally been proposed a transmissive mask using a membrane. However, there is a problem that since the absorption of the membrane for EUV light is large, the exposure time becomes long and thus high throughput cannot be achieved. Therefore, a reflective mask is generally used in the current state.
For example, Japanese Examined Patent Application Publication (JP-B) No. H7-27198 (Patent Document 1) or Japanese Unexamined Patent Application Publication (JP-A) No. H8-213303 (Patent Document 2) discloses a reflective mask for exposure in which a reflective layer having a multilayer film structure is formed on a substrate and an absorber adapted to absorb a soft X-ray or a vacuum ultraviolet ray is formed in a pattern on the reflective layer. FIGS. 1A and 1B are exemplary sectional views showing an example of such a conventional reflective mask blank for exposure and a conventional reflective mask for exposure. The reflective mask blank for exposure shown in FIG. 1A is configured such that a multilayer reflective film 101 having a multilayer film structure is formed on a substrate 100, an etching stopper layer 102 is formed on the multilayer reflective film 101, and an absorbent layer 103 is formed on the etching stopper layer 102. A pattern 103a is formed in the absorbent layer 103 of the reflective mask blank for exposure and the unnecessary etching stopper layer 102 on the multilayer reflective film 101 is removed along the absorbent layer pattern 103a, thereby fabricating the reflective mask for exposure shown in FIG. 1B. Exposure light such as a soft X-ray incident on the reflective mask for exposure is reflected at a portion where the multilayer reflective film 101 is exposed, while, is not reflected but is absorbed at a portion where the pattern 103a of the absorbent layer is formed. As a result, it is possible to form a pattern (optical image) with a high contrast between a reflected portion and an absorbed portion.
However, in the reflective mask for exposure having the multilayer reflective film 101 and so on formed on the substrate 100 as described above, it is necessary to increase the film density of each layer of the multilayer reflective film 101 in order to obtain a high reflectance. Consequently, the multilayer reflective film 101 inevitably has a high compressive stress. Because of this high compressive stress, the substrate 100 is largely warped (deformed) to thereby form a convex surface as shown in FIG. 2. As a result, the surface of the multilayer reflective film 101 serving as a reflecting surface for EUV light is also subjected to warping. For example, when a compressive stress of about 400 MPa is applied to a multilayer reflective film with a thickness of about 0.3 μm on a 6-inch square (152.4 mm×152.4 mm) quartz glass substrate with a thickness of 6.35 mm, warping (deformation) of about 1000 nm occurs in an area of 142 mm×142 mm.
The absorbent layer 103 and the etching stopper layer 102 formed on the multilayer reflective film 101 are each required to have a low stress close to zero because of being subjected to patterning and, therefore, the flatness of the mask blank is mainly controlled by the film stress of the multilayer reflective film. Generally, the reflective mask for EUV light is fixed by an electrostatic chuck during exposure. It has conventionally been considered that even if the mask is deformed as shown in FIG. 2, the flatness of the mask can be corrected by the electrostatic chuck. However, in the prior art, the mask surface is deformed into a convex surface by about 1 μm in the compression direction due to high compressive stress of the multilayer reflective film and thus the back surface of the mask (i.e. the surface on the side adsorbed by the electrostatic chuck) becomes a concave surface. In the case of the electrostatic chuck, the principle is such that adsorption spreads from a contact point and, therefore, the mask substrate is brought into contact therewith from its outer side and the adsorption spreads toward its inner side. FIGS. 3A and 3B each show the state where the mask blank (or the mask) is fixed (adsorbed) by the electrostatic chuck. Since the flatness of the mask blank is mainly controlled by the film stress of the multilayer reflective film, the film-coated substrate in which the multilayer reflective film 101 is formed on the substrate 100 is shown as an example herein for convenience of description. In each of FIGS. 3A and 3B, a diagram on the left side shows the state where the film-coated substrate is first brought into contact with an electrostatic chuck stage 60 and a diagram on the right side shows the state where the film-coated substrate is finally adsorbed. As shown in FIG. 3A, when the surface to be adsorbed by the electrostatic chuck is a concave surface like the conventional structure, there has been a problem that since outer-side contact points first brought into contact with the electrostatic chuck are fixed, a clearance at a center portion formed at the time of the first contact is not completely removed and thus adsorption failure occurs or, even if adsorbed, the substrate is not completely corrected (flattened).
For this problem, it is considered to reduce the stress of the multilayer reflective film 101. However, this reduces the film density and causes a reduction in reflectance of EUV light, and thus is not preferable from a practical viewpoint. In addition, a method is considered that improves the flatness using a stress correction film. However, even if the stress correction film is formed, it is technically difficult to completely correct the in-plane stress distribution and the film thickness distribution and, even if the flatness exhibits a certain flatness, the in-plane distortion is not completely removed and thus the problem of adsorption failure is not solved. Further, depending on a material used for the stress correction film, there is a case where the substrate is not flattened and waviness occurs. Even if attempting to fix the film-coated substrate having such waviness by the electrostatic chuck, the distortion is not removed, thus resulting in adsorption failure (see FIG. 3B). Further, it is not possible to avoid occurrence of a further change in film stress due to heat loads generated in the mask blank fabrication processes and the mask fabrication processes and due to a time-dependent change. In view of the foregoing, there has been a limit in adjusting to improve the flatness by the use of the stress correction film.